2 If there’s more water vapor than places for it to condense, already-formed ice crystals can also serve as seeds. As the crystals take on moisture, they may become too heavy for updrafts to support. Time for the umbrella.

8 In fact, a 2009 Stanford University study claims that clouds created by aircraft emissions triggered an overall rise in surface temperatures of 0.03 to 0.06 degree Celsius worldwide. That would account for 4 to 8 percent of the warming that has occurred since record keeping began in 1850.

9 Nacreous clouds, or “mother of pearl” clouds, appear iridescent because of their ultrafine ice crystals, which form 10 to 15 miles up in the stratosphere.

10 Unfortunately, nacreous clouds also support chemical reactions that convert benign chlorine-containing molecules into a form that destroys Earth’s ozone layer.

11 Roll clouds form when updrafts and downdrafts churn clouds into a long, spinning cylinder. They look spectacular, but they often herald an approaching storm front.

12 Highest of them all: 50 miles up, noctilucent, or “night shining,” clouds glow an eerie bluish white. They are invisible by day, but after sunset they catch solar rays shining from far below the horizon.

13 Noctilucent clouds seemed to first appear after the 1883 eruption of Krakatoa and are now a common sight.

16 Cumulonimbus clouds are the ones that make your flight late. Their winds are so intense and unpredictable that pilots never go through them.

17 Not “through” but sometimes over.

18 In 1959 Lt. Col. William Rankin was flying his F-8 fighter jet over a cumulonimbus when the engine failed. He parachuted out and spent the next 30 minutes bounced around inside the storm. Amazingly, he survived.

19 In 2007 German paragliding champion Ewa Wisnierska experienced “cloud suck.” While gliding under a cumulonimbus, she was pulled upward to 32,000 feet. She blacked out due to lack of oxygen but regained consciousness at roughly 23,000 feet.

20 Referring to the dark clouds on the horizon, Wisnierska said, “Usually there is no problem.”